He doesn't need to finish. Just have enough worked out to hand them an outline to base their scripts on.
Um. I hope, despite D&D knowing roughly where George is taking the story, they just do their own thing when they get to the ending, out of respect for the book readers to let us see the story end the way George intends it to end from fresh eyes. Just cause a couple people online spoil TV people doesn't mean the show itself has to spoil fans!